1. Field of the Invention
The present invention relates to an improvement in a fixing device for an image forming apparatus which is used to fix a toner image on an image forming medium such as a sheet of paper passed between a heating roller and a supporting roller.
2. Description of the Prior Art
A heating roller transfers heat to an image forming medium by direct contact in this type of fixing device. Thus, the heat conduction efficiency is better than in other means in which heat is transferred by elements at a distance, such as in a heater lamp system or flash lamp system, etc. Further, in the case of a heating roller, a sufficient amount of heat can be imparted to a toner image formed on the image forming medium in an instant (about 0.020-0.025 sec) even if the temperature in the vicinity of the image forming medium is low. In addition, the amount of power consumed is low, and the device can be made more compact. As a result, this type of device is in wide use as a fixing device in electronic copying machines, electronic printers and similar image forming apparatus.
Conventional fixing devices of this type have a structure in which a heater lamp such as a halogen lamp is provided in a roller body. Thus, since the heat transmission means between the roller body and heater lamp in the heating roller of a conventional fixing device is in the form of light, there is considerable loss of heat due to reflection at the roller body's inner peripheral surface and radiation of light from openings at opposite ends of the roller body.
Consequently, in the case of a high-speed electronic copying machine (40 sheets/minute or more for A4 size paper), for example, the maximum rated power usable by the heater lamp out of the rated power of 1.5 kW for the machine is around 1 kW. Thus, when continuous copying is effected, the heating roller temperature falls, leading to decreased quality of fixing performance.
In the case of a small, low-speed electronic copying machine (15 sheets/minute or less for A4 size paper), as the heating roller has a small heat capacity, its temperature changes at a rapid rate. Since the speed at which the heater lamp light is switched on and off is slower than this rate, the heating roller temperature displays a considerable amplitude of variation. When the heating roller temperature falls, the fixing performance falls. If the heating roller temperature rises too high, there can be problems of softening or deformation of fluororesin, constituting a parting layer on the heating roller's outer surface, and of resin elements, such as heat insulating bearings supporting the roller body and heat insulating gears supplying drive power to the roller body.